RE: [PATCH v4] vhost: support CPU copy for small packets

["Ma, WenwuX" <wenwux.ma@intel.com>]

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: 2022年9月7日 22:47
> To: Ma, WenwuX <wenwux.ma@intel.com>; maxime.coquelin@redhat.com;
> Xia, Chenbo <chenbo.xia@intel.com>; dev@dpdk.org; Richardson, Bruce
> <bruce.richardson@intel.com>
> Cc: Pai G, Sunil <sunil.pai.g@intel.com>; Hu, Jiayu <jiayu.hu@intel.com>;
> Wang, Yinan <yinan.wang@intel.com>; He, Xingguang
> <xingguang.he@intel.com>; Ding, Xuan <xuan.ding@intel.com>; Jiang,
> Cheng1 <cheng1.jiang@intel.com>; Wang, YuanX <yuanx.wang@intel.com>
> Subject: RE: [PATCH v4] vhost: support CPU copy for small packets
> 
> > From: Wenwu Ma [mailto:wenwux.ma@intel.com]
> > Sent: Monday, 29 August 2022 02.57
> >
> > Offloading small packets to DMA degrades throughput 10%~20%, and this
> > is because DMA offloading is not free and DMA is not good at
> > processing small packets. In addition, control plane packets are
> > usually small, and assign those packets to DMA will significantly
> > increase latency, which may cause timeout like TCP handshake packets.
> > Therefore, this patch use CPU to perform small copies in vhost.
> >
> > Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
> > ---
> 
> [...]
> 
> > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c index
> > 35fa4670fd..cf796183a0 100644
> > --- a/lib/vhost/virtio_net.c
> > +++ b/lib/vhost/virtio_net.c
> > @@ -26,6 +26,8 @@
> >
> >  #define MAX_BATCH_LEN 256
> >
> > +#define CPU_COPY_THRESHOLD_LEN 256
> 
> This threshold may not be optimal for all CPU architectures and/or DMA
> engines.
> 
> Could you please provide a test application to compare the performance of
> DMA copy with CPU rte_memcpy?
> 
> The performance metric should be simple: How many cycles does the CPU
> spend copying various packet sizes using each the two methods.
> 
> You could provide test_dmadev_perf.c in addition to the existing
> test_dmadev.c.
> You can probably copy a some of the concepts and code from
> test_memcpy_perf.c.
> Alternatively, you might be able to add DMA copy to test_memcpy_perf.c.
> 
> I'm sorry to push this on you - it should have been done as part of DMAdev
> development already.
> 
> -Morten

The test application has been supported by the following patch.

http://patchwork.dpdk.org/project/dpdk/patch/20220919113957.52127-1-cheng1.jiang@intel.com/